A spy who penetrated the IRA leadership on behalf of the British military has been named as Freddie Scappaticci by security sources.
The agent was a former leading IRA figure
The agent, codenamed Stakeknife, left Northern Ireland early on Sunday for his own safety, after he was warned on Friday his personal security had been compromised.
He is alleged to have been involved in dozens of murders in Northern Ireland.
A team of detectives investigating collusion between the security forces and paramilitaries have said they want to question him.
The agent is believed to have been at the top of the IRA in Belfast for several decades, and worked for the Army's Force Research Unit inside the republican organisation.
Scappaticci, now in his late fifties, was described by security sources as the "jewel in the crown".
He was publicly named on Sunday morning in a number of newspapers in the UK and Ireland.
Suzanne Breen, from the Irish Times, said: "It has sent ructions through republican districts in Northern Ireland.
"People cannot believe that the British Army managed to penetrate the Provisional IRA at the highest rank, and so many people have been killed, tortured and jailed on this man's actions."
Sean O'Callaghan, a former agent within the IRA, told the BBC: "I was very aware of the name.
"It was a name that would have struck fear into the hearts of most IRA people."
He said Scappaticci was one of a group of people who investigated IRA people for "any kind of imaginable offence".
They had a "ruthless" reputation, he said.
The Army has not commented on the reports, nor has anything been said by republicans.
Speaking on BBC Northern Ireland on Sunday, Ulster Unionist leader David Trimble said nobody in the province should be surprised by the revelations.
"We all know that the authorities try to penetrate the paramilitary organisations," he said.
"It is the key way in which the paramilitaries have been ground down and brought close to defeat in Northern Ireland.
"The authorities' job is to get intelligence, to get information, and that means turning people who are members of paramilitary organisations or finding someone who will penetrate them."
A few weeks ago, the Metropolitan Police Commissioner, Sir John Stevens, said his team of officers - investigating collusion between the security forces and loyalist paramilitaries - was proposing to interview Scappaticci.
There have been claims that loyalists were steered by army intelligence to murder other Catholics in order to protect Scappaticci from such an assassination.
The latest reports claim he was involved in murders himself but that the security forces believe he saved many more lives.
The revelations, if proven, would be hugely damaging for the Army.
A Northern Ireland Office spokeswoman refused to discuss the disclosure.
"We wouldn't comment on anything of an intelligence or security nature," she said.